Conventionally, as an intake air amount control system for an internal combustion engine, there is known one described in Patent Literature 1, for example. The engine is of a four-cylinder type, and is comprised of an electromagnetic valve mechanism provided for each cylinder, for changing the valve-closing timing relative to the valve-opening timing of each cylinder, and a throttle valve mechanism. As described hereinafter, in this intake air amount control system, to control idling speed, the valve-closing timing of each intake valve is controlled via the electromagnetic valve mechanism, whereby the intake air amount is controlled. More specifically, a target intake air amount for feedforward control is calculated according to target engine speed, and further, the maximum value of absolute values of the differences between an average value of valve-closing times of the intake valves of all the cylinders and the valve-closing times of the cylinders is calculated. Further, according to the maximum value of the absolute values of the differences, a gain for feedback control is calculated. A target intake air amount for feedback control is
calculated according to the gain, and the valve-closing timing of each intake valve is calculated based on the two target intake air amounts for feedback control and feedforward control and so forth, and the valve-closing timing of the intake valve is controlled by the calculated valve-closing timing. The valve-closing timing of each intake valve is controlled as described above, thereby suppressing variation in the idling speed caused by the difference in the intake air amount between the cylinders resulting from the different valve-closing times of the respective intake valves.
According to the conventional intake air amount control system, the target intake air amount for feedback control is calculated based the maximum value of the respective absolute values of differences between the average value of the valve-closing times of the intake valves of all the cylinders and the valve-closing times of the cylinders, and finally only one value is calculated as the valve-closing timing for the intake valves. Then, the intake valves of all the cylinders are only controlled using the one value. Therefore, there is a problem of incapability of correcting variation in the intake air amount between the cylinders. Therefore, in the control of the engine speed in a very low load region, such as an idling speed region, it is possible to suppress the rotational variation, but in a normal operation load region, torque variation or rotational variation occurs due to variation in the intake air amount between the cylinders, and the combustion state is degraded. As a consequence, drivability and exhaust emission characteristics are degraded. This problem becomes conspicuous particularly in a high load region or during lean operation (during execution of EGR).
The present invention has been made so as to solve the above problems, and an object thereof is to provide an intake air amount control system for an internal combustion engine, which is capable of correcting variation in intake air amount between cylinders, to thereby improve both drivability and exhaust emission characteristics even in a normal operation load region including a high load region.
[Patent Literature 1]
Japanese Laid-Open Patent Publication (Kokai) No. 2001-140661 (pages 5 and 6, FIGS. 6 to 18)